Bab 849: Soul Force + Authority.

The only way Celestials grow stronger is to imbibe more domains. They are unlike Origin gods and Demon kings who sacrifice concepts and Celestial Godhood respectively to grow their Authority. But the tree father won't remain in his stagnant situation for long. 

So Aeternus has reached the limit of the path for a demon king. He can't initiate his evolution yet because his evolution is not normal. He has to contend with primordial chaos to become a demon god. So he is waiting to fully comprehend the law of Chaos before he attempts the fusion with his mark of sin. 

The Godhoods he is acquiring are for the tree father to grow stronger since he doesn't need them anymore. He much prefers to empower himself with soul force.